Physicists at Rutgers University in New Jersey have discovered new properties in a material that could result in efficient and inexpensive plastic solar cells for electricity production. The discovery, posted online and slated for publication in an upcoming issue of the journal Nature Materials, reveals that energy carrying particles generated by packets of light can travel on the order of a thousand times farther in organic (carbon-based) semiconductors than scientists previously observed. This boosts scientists' hopes that solar cells based on this new type of technology may one day overtake silicon solar cells in cost and performance, thereby increasing the practicality of solar generated electricity as an alternate energy source to fossil fuels.

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All solar cells require a light absorbing material contained within the cell structure to absorb photons and generate electrons via the photovoltaic effect. The materials used in solar cells tend to have the property of preferentially absorbing the wavelengths of solar light that reach the Earth surface.

Many currently available solar cells are configured as bulk materials that are subsequently cut into wafers (silicon being the most prevalent bulk material). Other materials are configured as thin-films such as polymers that are deposited on supporting substrates, while a third group are configured as nanocrystals and used as quantum dots (electron-confined nanoparticles) embedded in a supporting matrix.

Silicon remains the only material that is well-researched in both bulk (also called wafer-based) and thin-film configurations.

"Organic semiconductors are promising for solar cells and other uses, such as video displays, because they can be fabricated in large plastic sheets," said Vitaly Podzorov, assistant professor of Physics at Rutgers. "But their limited photo-voltaic conversion efficiency has held them back. We expect our discovery to stimulate further development and progress."

The invention of conductive polymers (for which Alan Heeger, Alan G. MacDiarmid and Hideki Shirakawa were awarded a Nobel prize) may lead to the development of much cheaper cells that are based on inexpensive plastics. However, organic solar cells generally suffer from degradation upon exposure to UV light and may not be viable long term. Additionally, the conjugated double bond systems in the polymers which carry the charge, react more readily with light and oxygen. So most conductive polymers, being highly unsaturated and reactive, are highly sensitive to atmospheric moisture and oxidation, making commercial applications difficult.

Podzorov and his colleagues observed that excitons — particles that form when semiconducting materials absorb photons, or light particles — can travel a thousand times farther in an extremely pure crystal organic semiconductor called rubrene. Until now, excitons were typically observed to travel less than 20 nanometers — billionths of a meter — in organic semiconductors.

"This is the first time we observed excitons migrating a few microns," said Podzorov, noting that they measured diffusion lengths from two to eight microns, or millionths of a meter. This is similar to exciton diffusion in inorganic solar cell materials such as silicon and gallium arsenide.

"Once the exciton diffusion distance becomes comparable to the light absorption length, you can collect most of the sunlight for energy conversion," he said.

Excitons are particle-like entities consisting of an electron and an electron hole (a positive charge attributed to the absence of an electron). They can generate a photo-voltage when they hit a semiconductor boundary or junction, and the electrons move to one side and the holes move to the other side of the junction.

While the extremely pure rubrene crystals fabricated by the Rutgers physicists are suitable only for laboratory research at this time, the research shows that the exciton diffusion bottleneck is not an intrinsic limitation of organic semiconductors. Continuing development could result in more efficient materials.